1. Field of the Invention
The invention relates to a control apparatus and control method for an internal combustion engine equipped with a purge device that introduces fuel evaporative emission into an intake passage and that controls an air-fuel ratio to a rich air-fuel ratio when returning from fuel cut.
2. Description of the Related Art
A fuel cut control is known as an existing technique. In the fuel cut control, when a predetermined requirement is satisfied, for example, supply of fuel to all combustion chambers of an internal combustion engine is interrupted in order to improve fuel economy. During fuel cut control of this type, only air flows through a catalyst carrier of an exhaust emission control device. Thus, there is a possibility that the air may accumulate in the catalyst carrier and, furthermore, the catalyst carrier temperature may decrease as compared with an active temperature. For this reason, when supply of fuel is resumed (that is, when returning from fuel cut), the purification performance of the exhaust emission control device for exhaust gas may possibly be decreased. Then, in an existing technique, in order to promptly consume air accumulated in the catalyst carrier and to quickly increase the catalyst carrier temperature to the active temperature when the catalyst carrier temperature is lower than the active temperature, the air-fuel ratio is controlled so that the actual air-fuel ratio temporarily attains a rich air-fuel ratio when returning from fuel cut (hereinafter, referred to as “fuel cut return rich control”). The above fuel cut return rich control is, for example, described in Japanese Patent Application Publication No. 2005-105834 (JP-A-2005-105834).
Incidentally, in a fuel tank, fuel evaporative emission (evaporation gas) is generated because of evaporation of stored fuel. Therefore, as the fuel evaporative emission increases, the internal pressure of the fuel tank increases. For this reason, an internal combustion engine generally includes a purge device that introduces (purges) fuel evaporative emission in the fuel tank into an intake passage to release the increased internal pressure. The purge device monitors a pressure in the intake passage, to which fuel evaporative emission is supplied, with a pressure sensor, and performs purge control such that fuel evaporative emission is introduced into the intake passage when the monitored pressure satisfies a predetermined requirement (for example, when the pressure in the intake passage attains a pressure corresponding to a predetermined intake negative pressure of the internal combustion engine).
Here, in a typical air-fuel ratio control of an internal combustion engine, the fuel supply rate from a fuel injector is adjusted in accordance with an intake air flow rate to achieve a target air-fuel ratio. For this reason, when the purge control is performed, the actual air-fuel ratio deviates to a rich side with respect to the target air-fuel ratio by the rate of introduced fuel evaporative emission (that is, purge flow rate). In addition, if the purge control is performed when no fuel evaporative emission is present in a path through which fuel evaporative emission is supplied, only air is supplied to the intake passage. Thus, the actual air-fuel ratio in this case deviates to a lean side with respect to the target air-fuel ratio. Therefore, in the existing technique, the purge flow rate is controlled so as to perform purge control while suppressing a large deviation between an actual air-fuel ratio and a target air-fuel ratio. For example, the purge flow rate is varied on the basis of a pressure in the intake passage.
However, when the value detected by the pressure sensor is abnormal, the actual purge flow rate increases or decreases with respect to a target purge flow rate at the time when the detected value is normal. Thus, the actual air-fuel ratio largely deviates to a rich side or a lean side with respect to the target air-fuel ratio. Then, particularly, if the actual air-fuel ratio during fuel cut return rich control becomes a lean air-fuel ratio, the amount of nitrogen oxides (NOx) generated in the combustion chambers increases. Moreover, it is difficult to promptly activate the exhaust emission control device, which is a main purpose of the fuel cut return rich control. This may lead to deterioration in emission performance due to poor purification of NOx in exhaust gas.